1. Field of the Invention
The present invention relates to a method and an apparatus for magnetic resonance imaging.
2. Description of the Prior Art
Various magnetic resonance imaging techniques are known within the category designated “parallel imaging.” These techniques have in common the use of multiple data acquisition (RF) coils, that each acquire, either simultaneously or substantially simultaneously, a set of magnetic resonance data. Each coil acquires a reduced set of k-space data, with no single set of k-space data, acquired by one coil being sufficient for generating a complete image of the examination subject. The data acquisition is thus accelerated, and the time required for obtaining all of the data necessary to generate an image of the subject is reduced. The respective data sets acquired by the multiple coils are combined in an appropriate manner using information about the individual coils, such as their location or sensitivity.
Despite the advantage achieved by shortening the time for acquiring the necessary data for generating an image of the subject, parallel imaging has the disadvantage associated therewith that convolution artifacts can arise in the image. The reason for this is schematically illustrated in FIG. 1. The sub-sampling in k-space leads to a reduced field of view (FOV) in image space (image domain). If the examination subject is larger than the reduced FOV, this leads to a convolution of the regions of the subject that lie outside of the reduced FOV, resulting in convolution artifacts in the image.
In parallel imaging, either the missing k-space data are reconstructed (as in the sequence known as GRAPPA) or the convoluted images are deconvoluted (as in the sequence known as SENSE). The primary problem in parallel imaging is image disruptions that arise due to inherent errors in the reconstruction. These artifacts occur primarily with the use of high acceleration factors, typically acceleration factors greater than two. Remaining convolution artifacts occur in k-space-based methods (such as GRAPPA) has shown in image (c) in FIG. 4), and noise amplification in the image occurs in image space-based methods (such as SENSE).
In addition to parallel imaging, imaging methods known as zoomed methods known are known that also allow an accelerated data acquisition. The basic features of the known zoomed technique is shown in FIG. 2. A reduced FOV is acquired in the zoomed method, similar to that in parallel imaging. In order to avoid convolution artifacts in the reduced FOV, either the signals from the outer regions are suppressed with saturation pulses emitted in advance (known as the “outer volume suppression” or “OVS” method), or only the region inside the reduced FOV is excited (“inner volume excitation” method). In both of these methods, convolution artifacts still can occur, due to an imperfect saturation of the outer regions in the OVS method, as shown in image (d) of FIG. 4 and the middle image in FIG. 5, or by an imprecisely (fuzzy) demarcated excitation of the region within the reduced FOV in the inner volume excitation method.
These drawbacks associated with these known types of imaging sequences have previously been addressed in the case of parallel imaging by a recommendation from apparatus manufacturers either not to use parallel imaging for certain applications, or to use parallel imaging with only a relatively small acceleration factor, such as an acceleration factor that is not greater than two. Problematic uses of parallel imaging include fMRI and spine imaging.
Resort to the zoomed method cannot be undertaken by default, because a patent exists with regard to the inner volume excitation method (GB 2 205 410 A).
The zoomed method was first described in 1988 in the context of echo planar imaging (EPI), in the article by Mansfield et al entitled “Zonally Magnified EPI in Real Time by NMR”, J. Phys. E. Sci. Instrum, Vol. 21, (1988), pages 275-280). Zoomed EPI for ultra-high field fMRI at 7 Tesla is described in the article by Pfeuffer et al., “Zoomed Functional Imaging in the Human Brain at 7 Tesla with Simultaneous High Spatial and High Temporal Resolution” NeuroImage, Vol. 17 (2002) pages 272-286. Zoomed EPI for DTI on the optic nerve is described in the article by Kingshott et al, “In Vivo Diffusion Tensor Imaging of the Human Optic Nerve: Pilot Study in Normal Control,” Magnetic Resonance in Medicine, Vol. 56 (2006), pages 446-451.
The problems described above in connection with zoomed methods still exist.